Optical information recording media such as optical discs and optical-magnetic discs have been widely used in the past as data recording media and audio information recording media. In recent years, CD-R, CD-RW, DVD-R and DVD-RW, etc., have also begun to be used in addition to such optical information recording media. In these optical information recording media, variations in reflectivity caused by marks such as fine pits formed in the surface of a disc-form recording medium, or variations in characteristics caused by the different magnetic marks recorded on the magnetic film formed on the surface of such a disc-form recording medium, are utilized as information, and the recording of information is accomplished by utilizing optical means.
In regard to the meanings of technical terms relating to optical information recording media that use such systems, some of these terms are described in JISX6261 “130 mm Writable Optical Disc Cartridges” and JISX6271 “130 mm Rewritable Optical Disc Cartridges”; accordingly, in the present specification, the technical terms described in these publications will be used in the senses described in these publications unless otherwise noted.
In these optical information recording media, meandering grooves and lands are alternately formed in a spiral pattern, and information is ordinarily written in the grooves. Furthermore, the grooves and lands are used for position control, i.e., position detection for the purpose of tracking, which causes the pickup of the recording device or playback device to run along the zone in which information is written. Specifically, since the intensity of the reflected light varies according to the position of the groove or land that corresponds to the position illuminated with light, a control action is performed in which the recording device or playback device receives the signal, and the position of the pickup is controlled so that the positions in which information is written are accurately illuminated with light.
Furthermore, among these optical information recording media, a standard known as the “DVD book” has been established in the case of recordable DVD (DVD-R, DVD-RW). According to this standard, pre-embossed pits known as “land pre-pits (LPP)” are regularly arranged in the land parts adjacent to the spiral-form meandering grooves of the disc-form optical information recording medium (hereafter referred to simply as a “disc” in some cases).
Control information, etc., such as disc address information, disc manufacturer information and disc writing information, which is sent to the recording device or playback device, etc., when information is recorded on the optical information recording medium or when information is read out from the optical information recording medium is embedded beforehand in the land pre-pits.
At the same time that the recording and playback device records or plays back information in the groove regions of the DVD-R disc or DVD-RW disc while tracking these groove regions by means of a light spot, the recording and playback device also plays back the land pre-pit signals from the adjacent land and thus acquires necessary information. For example, in the case of a DVD-R disc, recording marks are written with a light spot while the grooves are tracked during recording; at the same time, address information, etc., of the land pre-pits is played back. Furthermore, in cases where a recorded DVD-R disc is played back by the recording and playback device, the adjacent land pre-pits are played back while the pits that have been recorded in the groove region are played back.
The length itself of the land pre-pits that are formed is not fixed by any standard; however, especially in cases where it is necessary to play back land pre-pits during recording, a certain minimum strength is required by standards for the signal strength during land pre-pit playback. Meanwhile, in the case of DVD-R and DVD-RW discs, the length of the marks that are recorded is fixed at 0.4 μm for the minimum mark length by the DVD Book standard. However, in the case of DVD-R and DVD-RW discs, a system is used in which the address information of the land pre-pits is played back while recorded marks written in the grooves are played back; accordingly, in cases where regions in which minimum marks of 0.4 μm are written are played back, there is a danger that the minimum mark playback signals and land pre-pit playback signals will interfere with each other if the length of the land pre-pit signals is 0.4 μm or greater. As a result, it has been found that this causes problems in terms of an increase in recorded mark jitter and an increase in the error rate.
Furthermore, in cases where marks that are longer than the minimum marks are played back as well, the marks are recognized as being longer than the marks that are actually written because of interference with the land pre-pit signals; as a result it has similarly been found that this leads to problems in terms of an increase in jitter and an increase in the error rate.
In the case of DVD-R and DVD-RW discs, as was described above, the formation of the land pre-pits with an optimal length and shape is extremely important when these land pre-pits are played back by the recording and playback device.
Meanwhile, if the stability of the land pre-pit playback signal quality is taken into account, it is desirable that the respective land pre-pits completely cover the width of the lands, and that the ratio of the minimum value of the length to the maximum value of the length be equal to or greater than a specified value. If there are portions in which land pre-pits are not formed in the direction of width of the lands, this will cause variation in the read-out signal strength of the land pre-pits, thus leading to a drop in the signal quality. Similarly, if the ratio of the minimum value of the length of the land pre-pits (0 in a case where there are portions in which land pre-pits are not formed in the direction of width of the lands) to the maximum value of this length is less than a specified value, this results in variation in the read-out signal strength of the land pre-pits, thus leading to a drop in the signal quality.
However, it has been found that such shortening the length of the land pre-pits in order to prevent an increase in recorded mark jitter and an increase in the error rate, covering the entire width of the lands with the respective land pre-pits in order to achieve stability of the land pre-pit signals, and devising the system so that the ratio of the minimum value of the length to the maximum value of the length is a specified value or greater, are in a trade-off relationship if conventional techniques are used, so that these objectives cannot be simultaneously achieved.
Specifically, in the case of optical discs such as DVD-R and DVD-RW, substrate duplication is performed from a mold called a stamper via an injection molding process. The land pre-pits are also formed by a master plate for stamper exposure process in the stamper manufacturing process.
In this master plate for stamper exposure process, the light spot of the laser cutting machine has a circular shape; accordingly, the shape of the land pre-pits is also circular. Consequently, if this diameter is increased in order to increase the land pre-pit signal strength, the land pre-pits cover the entire width of the lands, and the ratio of the minimum value of the length to the maximum value of the length is increased, so that the land pre-pit signal quality is increased; however, the length of the land pre-pits is also correspondingly increased, so that the land pre-pits interfere with the marks recorded in the grooves, thus lowering the quality of the recorded mark playback signal.
Conversely, if the length of the land pre-pits is shortened in order to improve the recorded mark playback signal quality, the respective land pre-pits cannot cover the entire width of the lands since the light spot that is used to form the land pre-pits is circular. Furthermore, the ratio of the minimum value of the length to the maximum value of the length is reduced, thus causing a drop in the land pre-pit signal strength.